Electrical surges such as electrical overstress or electrostatic discharge (ESD) transient pulses are common causes of damage to electronic devices. To protect against such transient surges electronic devices are conventionally protected by surge or ESD protection devices. One type of protection device is the so called Transient voltage suppression (TVS) device.
TVS devices provide protection against electrical overstress or electrostatic discharges and are commonly used in portable/consumer electronic devices such as personal computers, audio and video equipment or mobile telephones. According to the International Electrotechnical Commission standard IEC 61000-4-2 such devices should be protected against system level ESD stresses.
Where protection is required at a system level, for example in a portable electronic device such as a smart phone or tablet computer ESD protection, devices must be adequately protected in accordance with the IEC standards whilst not hampering normal operation of the device. In applications with high speed interfaces such as Universal Serial Bus (USB) and High Definition Multimedia Interface (HDMI) it is necessary that ESD devices have low device capacitance so that signal integrity is maintained. Such ESD devices also require a low clamping voltage in order to protect internal circuitry.
The requirement of low clamping voltage is related to the gate oxide thickness utilised in integrated circuits and devices used in electronic applications. The gate oxide is the dielectric layer that separates the gate terminal of a MOSFET from the underlying source and drain terminals as well as the conductive channel that connects source and drain when the transistor is turned on. Gate oxide is formed by oxidizing the silicon of the channel to form a thin (5-200 nm) insulating layer of silicon dioxide.
A conductive gate material is subsequently deposited over the gate oxide to form the transistor. As device and integrated circuit miniaturization continues the gate oxide thickness must be reduced accordingly. This reduction of gate oxide thickness can reduce the breakdown voltage of the device or IC.
By having a low clamping voltage it is possible to ensure that the gate oxide breakdown voltage is not exceeded. That is, it is possible to set the clamping voltage at an appropriate level to ensure that it does not exceed the gate oxide breakdown voltage. Typically the clamping voltage is set such that it is equal to the maximum voltage drop across the protection device during an ESD or overstress event.
The requirement of low capacitance is related to the high data transfer rates. If the capacitance of the ESD protection device connected to the high data transfer lines is too high the signal may be distorted and data transfer may be reduced or prevented.
Commonly, diodes such as zener diodes are used to provide surge or overstress protection. Whilst such diodes are easy to manufacture and provide a cost effective protection against surge or overstress they have a high capacitance typically in the range of one to several hundred Pico-farads. Due their high capacitance zener diodes are therefore unsuitable to high data rate applications.
So-called silicon controlled rectifiers (SCR) may also be used for on-chip protection however they are not used as discrete protection. Typically the ESD robustness is very different depending on the required application. Normally an internal (or on-chip) ESD protection only protects against overstress events during manufacturing. Discrete protection on the other hand protects devices during operation of a system, such as a HDMI or USB data transfer line. Furthermore SCRs used for surge protection in on-chip or integrated circuit (IC) applications are lateral devices of the type shown in FIG. 1, with contacts solely on the top of the device. Such arrangements are therefore not suitable for standard discrete packages such as SOD882 for example. Specifically, standard discrete packages are generally miniaturised and a lateral device with two top terminal contacts may be very difficult to assemble for a comparable miniaturised form factor.
It is an object of the present invention to provide an ESD protection device which solves or mitigates some or all of the above mentioned problems.